game is insanely optimized, I squeezed every last fps out of it, atm there is not much room for optimisation anymore except degrading visual quality
Well typical causes of framerate drops are:
-> Excessive vertex / polygon count. This causes a lot of extra shadow casts and draw calls to be performed. Try to use more skyboxes and flat-textures for distant stuff, rather than meshes.
-> Excessive usage of virtual RAM, such as EMS and XMS. If the HDD or SSD is being accessed at 100% rate, then its a pretty good sign that too much virtual RAM is being used.
-> Unnecessarily large texture files for the objects in an area. 8k textures tend to the largest things would ever need to be, unless you can zoom super far in to see the extra detail... but even still, you should be using mipmaps, so that 8k texture isn't viewed constantly at any draw distance. But most standard environmental details like rocks and grass shouldn't be larger than 1k. Skyrim for example, gets away with 0.25k, 0.5k and some 1.0k textures for most environmental details.
-> Level of Detail, proper use of Draw Distances and Mipmaps can help as well.
-> Optimizing the terrain geometry, will help to reduce pathfinding costs and thus reduce the AI costs. AI also is an expensive process.
-> Avoiding unnecessary physics calls, such as giving every entity a bit of cloth physics, or hair physics. Only give important things physic calls.
-> A profiler would help to tell you where your bottlenecks are. Generally, having more than 10,000 draw calls PER frame, will cause all but the best machines to be brought to their knees. 15,000 or more, and you'll be having even the best machines brought to their knees.
-> Being careful with the usage of specific UE4 special effects. For example, particle generators. Each one can produce millions of particles, but each generator is costly to use. So, if you are using tons of generators, it will obliterate the framerate.